Heretofore, as an ion exchange membrane to be used for salt electrolysis, a membrane made of a fluoropolymer having carboxylic acid type functional groups or sulfonic acid type functional groups has been known. Such a fluoropolymer is obtained by copolymerizing a fluoromonomer having a carboxylic acid type functional group such as a perfluorovinyl ether and a fluoroolefin such as tetrafluoroolefin (CF2═CF2).
As the copolymerization method, emulsion polymerization, suspension polymerization, solution polymerization or the like may be mentioned. Emulsion polymerization is carried out in an aqueous medium in the presence of a polymerization initiator and a perfluorocarboxylic acid type emulsifier (such as C7F15CO2NH4 or C8F17CO2NH4), but in recent years, there is concern about influences of such a perfluorocarboxylic acid type emulsifier on the environment.
Further, it has been known that in solution polymerization, as a polymerization medium, a perfluorocarbon such as perfluoromethylcyclohexane, 1,1,2-trichloro-1,2,2-trifluoroethane (CCl2F—CClF2) etc. can be used (for example, Patent Documents 1 and 2). However, such a solvent is likely to destroy the ozone layer in the atmosphere, and accordingly their use is limited.
On the other hand, a process has been proposed to produce a fluoropolymer by copolymerizing a vinyl ether having a functional group capable of being converted to a carboxylic acid group or a sulfonic acid group and tetrafluoroethylene using as a medium a hydrofluoro ether having small ozone depletion potential and small global warming potential (for example, Patent Document 3). However, a process for more efficiently producing a fluoropolymer useful as an ion exchange membrane, etc. has been desired.    Patent Document 1: JP-A-52-28586    Patent Document 2: JP-B-1-24171    Patent Document 3: JP-A-2005-29704